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Taste. PSY 295 – Sensation & Perception Christopher DiMattina , PhD. Taste keeps us alive. Sweet substances often have nutritive or caloric value Bitter substances are often poisons Salt is important physiologically. Flavor. F lavor of is a combination of its smell and taste sensations
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Taste PSY 295 – Sensation & Perception Christopher DiMattina, PhD
Taste keeps us alive • Sweet substances often have nutritive or caloric value • Bitter substances are often poisons • Salt is important physiologically PSY 295 - Grinnell College - Fall 2012
Flavor • Flavor of is a combination of its smell and taste sensations • Molecules travel into nasal cavity via the retro-nasal passage PSY 295 - Grinnell College - Fall 2012
Localization • Where do we feel foods localized? • What are some exceptions (which prove the rule) ? PSY 295 - Grinnell College - Fall 2012
Experiment • Eat something flavorful while holding your nose • Note that you cannot really discern the flavor well PSY 295 - Grinnell College - Fall 2012
The organ of taste PSY 295 - Grinnell College - Fall 2012
The organ of taste PSY 295 - Grinnell College - Fall 2012
Basic taste sensations • Thousands of different smells • Only four basic taste sensations - Sweet, Salty, Bitter and Sour • Each has different molecular basis PSY 295 - Grinnell College - Fall 2012
Today’s lecture • Anatomy and physiology • Basic tastes and their molecular basis • Survival and pleasure • Coding taste • Genetic variation in taste perception PSY 295 - Grinnell College - Fall 2012
Physiology of taste PSY 295 - Grinnell College - Fall 2012
Taste receptors live on tongue • Taste buds on tongue live in structures called papillae • Taste buds contain numerous taste cells • Different taste cells are ‘tuned’ to different chemicals PSY 295 - Grinnell College - Fall 2012
Papillae • Different kinds of papillae at different locations on tongue • Mostly arranged in a oval on sides and back of tongue PSY 295 - Grinnell College - Fall 2012
Kinds of papillae • Filiform – front of tongue, no taste function PSY 295 - Grinnell College - Fall 2012
Fungiform • Larger, also in front of tongue • Contain about 6 taste buds each • Individual variation in density PSY 295 - Grinnell College - Fall 2012
Foliate papillae • On sides of tongue, look like folds PSY 295 - Grinnell College - Fall 2012
Circumvallate papillae • In a row on back of tongue • Taste buds lie in moats PSY 295 - Grinnell College - Fall 2012
Also taste buds on roof of mouth • Demo in book: Dip finger in salt and slide along roof of mouth • Where hard and soft palates meet will feel a rush of saltiness • Can also try with sugar PSY 295 - Grinnell College - Fall 2012
Taste buds • Live in the papillae and contain taste receptor cells PSY 295 - Grinnell College - Fall 2012
Taste receptor cells • Microvilli extend into taste pore and contain the sites which bind molecules PSY 295 - Grinnell College - Fall 2012
Taste receptor cells • Come in three varieties • Two of these varieties do not form classical synapses on taste nerve – Type II release ATP diffusely on to taste nerve fibers PSY 295 - Grinnell College - Fall 2012
Taste receptor cells • Replaced after a few days • Different cells contacted by same nerve fiber show similar taste preferences (sweet, salty, etc…) PSY 295 - Grinnell College - Fall 2012
Systems level anatomy • Tongue innervated by three cranial nerves (IX, X, VII) • Project to nucleus of the solitary tract in the brainstem (medulla) PSY 295 - Grinnell College - Fall 2012
Systems level anatomy • NST project to thalamus, which projects to primary taste cortex • Secondary taste cortex is the orbitofrontal cortex PSY 295 - Grinnell College - Fall 2012
Orbital frontal cortex • Responsible for assigning hedonic value to stimuli • Receives projections from several modalities PSY 295 - Grinnell College - Fall 2012
Orbital frontal cortex • Encodes the value of foods • As monkey eats, neurons fire less and monkey eats less PSY 295 - Grinnell College - Fall 2012
Basic tastes PSY 295 - Grinnell College - Fall 2012
Molecular mechanisms • Two mechanisms • Salty and sour tastes are detected by cation channels • Sweet and bitter compounds detected by membrane spanning receptor proteins coupled (coupled to G-proteins) PSY 295 - Grinnell College - Fall 2012
Salty • NaCl is salt • Channels selective for Na+ • Sodium influx depolarizes cell PSY 295 - Grinnell College - Fall 2012
Sour • Sour substances are acidic • Acids contain hydrogen ions (H+) • Channels selective for these ions PSY 295 - Grinnell College - Fall 2012
Bitter • Thousands of compounds are bitter, many (most?) poisonous • Multi-gene family with about 25 bitter receptors • Many receptors respond to multiple bitter compounds PSY 295 - Grinnell College - Fall 2012
Bitter • Pregnant women more sensitive to bitter foods • Bitter receptors in gut slow absorption PSY 295 - Grinnell College - Fall 2012
Sweet • Only three different G-protein coupled receptors • Two form a heterodimer sensitive to both smaller and larger molecules – some proteins can be sweet PSY 295 - Grinnell College - Fall 2012
Umami is controversial • MSG (amino acid derivative) • Glutamate receptors in tongue. Suggested that this taste codes proteins • However many protein containing foods do not have MSG • Not everyone likes it (not hardwired) • Protein receptors in gut PSY 295 - Grinnell College - Fall 2012
Taste, survival and pleasure PSY 295 - Grinnell College - Fall 2012
Taste • Contrasts with olfaction • Taste recognizes a few basic molecules we need to survive • Bitter avoids poisons • Sour helps us avoid overly acidic substances • Sweet helps us find energy • Salty helps us regulate salt levels • Unlike smell, taste responses hard-wired PSY 295 - Grinnell College - Fall 2012
Studies in infants • Infants make stereotyped responses to different tastes PSY 295 - Grinnell College - Fall 2012
Specific hungers theory • Theory that we are driven to choose foods which contain things we need • Crave salt after we sweat, sweets when our blood sugar is low, etc… • However, is not valid for nutrients • Rats deprived of Vitamin B-12 • Given choice of new diet containing B-12 and switched • Also given choice of new diet NOT containing B-12 and also switched PSY 295 - Grinnell College - Fall 2012
Nutrient must be detectable • Receptors for sugar, salt, etc… • No receptors for vitamins in vegetables PSY 295 - Grinnell College - Fall 2012
Fat and sugar • In the ancestral environment where food was scarce these were highly rewarding since they provided many calories • Conditioned preference for sensory properties of foods containing fat • In the modern environment, our evolved cravings make us fat PSY 295 - Grinnell College - Fall 2012
Coding of taste PSY 295 - Grinnell College - Fall 2012
Labeled line • Different nerve fibers contact different kinds of taste cell receptors • Pattern of activity across fibers determines taste sensation PSY 295 - Grinnell College - Fall 2012
Monkey nerve fibers PSY 295 - Grinnell College - Fall 2012
Rat across fiber responses PSY 295 - Grinnell College - Fall 2012
Specificity coding • Knocked human bitter PTC receptors into mice which don’t usually express these receptors • Mouse with receptor knock-in avoids PTC, normal mouse does not • Analogous results when you knock out receptors for other bitter compounds PSY 295 - Grinnell College - Fall 2012
Individual variability PSY 295 - Grinnell College - Fall 2012
PTC and PROP • Some people taste these compounds (bitter), some do not • Gene expresses PTC/PROP receptors • Those with two recessive alleles are non-tasters, those with one or both dominant alleles are tasters PSY 295 - Grinnell College - Fall 2012
Tasters are heterogeneous lot • Some find taste of PTC/PROP mildly bitter • Some find it extremely bitter (super-tasters) PSY 295 - Grinnell College - Fall 2012
Supertasters PSY 295 - Grinnell College - Fall 2012
Density of fungiform papillae • To be a super-taster, you not only must have genes but also dense fungifom papillae PSY 295 - Grinnell College - Fall 2012
Supertasters • Those with most fungiform papillae have the most intense taste sensations in general • Most burn from hot chili peppers since these fungiform papillae also contain pain receptors PSY 295 - Grinnell College - Fall 2012
